1. Field of the Invention
The present invention relates to an oscillometric-type blood pressure measuring apparatus and particularly to such a blood pressure measuring apparatus comprising a cuff including two inflatable bags.
2. Related Art Statement
Generally, an oscillometric-type blood pressure measuring apparatus includes a cuff which is adapted to be wound around a body portion of a living subject and includes an inflatable bag; a pressure changing device which increases a pressing pressure of the inflatable bag up to a prescribed pressure value which could completely exclude blood from an artery present under the cuff, and subsequently slowly decreases the pressure in the bag at a prescribed rate; a pressure sensor which continuously detects the pressure in the bag during the slow deflation of the bag; a pulse-wave filter which extracts a pulse wave from the pressure in the bag, continuously detected by the pressure sensor; and a means for determining, as a blood pressure of the subject, a pressure in the bag at a time of occurrence of a prescribed change of respective amplitudes of successive heartbeat-synchronous pulses of the extracted pulse wave.
More specifically described, the above-indicated blood pressure measuring apparatus determines, as a systolic blood pressure of the subject, a static pressure of the inflatable bag at a time of detection of a rising point where the respective amplitudes of successive heartbeat-synchronous pulses of the pulse wave continuously detected during the slow decreasing of the pressure of the bag, significantly largely increase.
However, the blood pressure measuring apparatus has the problem that since only the single inflatable bag is provided in the cuff and accordingly the rising point of the amplitudes of the pulse wave, successively determined from the change of pressure in the bag, is not clear, the determined systolic blood pressure of the subject may not be accurate. The reason is that even if the pressing pressure of the cuff may be higher than the systolic blood pressure of the subject, as the pressure of the cuff approaches the systolic blood pressure, the pulsation of the artery under the cuff starts under a proximal or upstream end portion of the cuff and propagates to the cuff. In particular, in the case where the body portion, such as ankle, around which the cuff is wound is difficult to completely exclude the blood from the artery thereof, the pulsation of the artery is likely to be large even if the pressure of the cuff may be higher than the systolic blood pressure of the subject. Therefore, the rising point of the amplitudes of the pulse wave is likely to be unclear.
To solve the above-indicated problem, it has been proposed to provide a blood pressure measuring apparatus that measures a blood pressure using a cuff including two inflatable bags one of which is used for excluding blood from an artery and the other of which is used for detecting a pulse wave from the artery. Japanese patent document No. 5-269089 discloses an example of the blood pressure measuring apparatus. The apparatus disclosed by the Japanese patent document employs an inner cuff as an inflatable bag for detecting a pulse wave, the inner cuff being provided on substantially the middle portion of the inner surface (i.e., body-side surface) of an outer cuff as an inflatable bag for excluding blood. Therefore, even if the arterial pulsation may resume around the upstream end portion of the outer cuff, the pulsation does not directly propagate to the inner cuff, so that the amplitudes of the pulse wave detected from the inner cuff show a clear rising point, which contributes to improving the accuracy of measurement of systolic blood pressure of the subject.
However, even in the case where a pulse wave is detected by a pulse-wave detecting inflatable bag that is provided independent of a blood excluding inflatable bag, respective amplitudes of heartbeat-synchronous pulses of the pulse wave may not show a clear rising point, because the pulsation of an artery that propagates to the blood excluding bag in the state in which the pressing pressure of the blood excluding bag is higher than a systolic blood pressure of a living subject, further propagates from the blood excluding bag to the pulse-wave detecting bag. Therefore, even if a systolic blood pressure may be determined based on the change of respective amplitudes of heartbeat-synchronous pulses of the pulse wave detected by the pulse-wave detecting bag, the determined systolic blood pressure may not be accurate.
The apparatus disclosed by the above-indicated Japanese patent document employs an oscillation shielding plate between the blood excluding bag and the pulse-wave detecting bag, for preventing the pulsation of artery from propagating from the blood excluding bag to the pulse-wave detecting bag. In addition, since a tube (or a pipe) employed for supplying a liquid (i.e., an inflating fluid) to the pulse-wave detecting bag is branched from a tube (or a pipe) employed for supplying the liquid to the blood excluding bag, the pulsation of artery that propagates to the blood excluding bag may further propagate via the two tubes to the pulse-wave detecting bag. To solve this problem, the tube for supplying the liquid to the pulse-wave detecting bag is thinner than the tube for supplying the liquid to the blood excluding bag. Since, however, the arterial pulsation that propagates to the blood excluding bag partly propagates via the two tubes to the pulse-wave detecting bag, the amplitudes of pulses of the pulse wave detected through the pulse-wave detecting bag may not show a sufficiently clear rising point.